It is common to store water in unpressurized above-ground vessels from which water from time to time is drawn by means of pump or gravity flow, particularly where the water supply is intermittent, or where the instantaneous demand exceeds the flow rate of the source of supply. It is particularly desirable, to have a vessel which is sealed against egress or ingress of liquids when potable water is being stored.
Typical water storage tanks that are associated with dwellings and other smaller buildings will have 500 to 1000 liter capacity. A squat cylindrical shape tank is popular for reason of compactness and manufacturability. A representative 500 liter tank might be about 100 cm in diameter and 100 cm high. One approach is to make a one piece plastic tank, such as by blow molding or rotational molding. Such tanks have good integrity but owing to their bulkiness, the cost of shipping from factory to point of use can be high.
The present invention is concerned with plastic tanks that principally are intended for use when oriented as vertical cylinders, including tanks which are much larger than just mentioned. For instance, plastic tanks may have capacities of in the range of about 9,500 liters to about 11,000 liters or more. A representative tank may have a diameter of about 8 feet (2.4 meters), a height of about 8 to 10 feet (2.4 to 3.1 meters), a basic wall thickness of 0.19 to 0.4 inches (4.8 to 10 mm) and a weight of about 250 to 400 pounds (114 to 180) kilograms. If such tanks can be fabricated by mating two bucket-like parts and which are then welded to each other at or near the point of use, that can be a practical solution when the parts have sufficient draft to enable nesting for shipment.
A related commonly owned patent application bearing Ser. No. 15/413,251 and entitled “Making a plastic tank with an integrity-verifiable weld joint,” filed on even date herewith, describes how such a tank can be fabricated by welding together two molded bucket-like parts at a circumferential flange. Because the parts being mated are large, they can be subject to slight manufacturing variations (e.g., those related to variations in cooling of hot molded plastic), and possible distortion during handling or temporary storage or uneven sunlight heating, etc.
Thus, if the joint does not have good fit-up, there is a chance for unsoundness of the resultant weld. Generally, it is an objective to avoid workers having to spend time fitting parts together or to provide fixtures aimed at overcoming fit-up problems. One approach with such large tank-flange joints is to use a plurality of pins or dowels, which extend from vicinity of one flange into close-fitting holes in the mating part. An example is described in connection with FIG. 3 of U.S. Pat. No. 8,740,005 of Holbrook et al. See also U.S. Pat. No. 5,878,907 of Graf. The prior art tanks have a mid-elevation joint which is alternatively secured by means of clamps or by means of welding.